1. Field of the Invention
This invention relates to a high-temperature electrically regenerable electrochemical system. It patricularly relates to a lithium alloy-molten salt-transition metal chalcogenide secondary cell or battery providing long cycle life and high energy density. It specifically relates to a positive electrode structure for use in such devices.
2. Prior Art
High energy density batteries are of particular interest for application as a source of power for an electric vehicle and for load leveling in the electric utility industry. Initially, the interest was directed toward the lithiumsulfur cell using a molten halide; see M. L. Kyle et al, "Lithium/Sulfur Batteries for Electric Vehicle Propulsion", Sixth Intersociety Energy Conversion Engineering Conference Proceedings, p. 38 (1971); and L. A. Heredy et al, Proc. Intern. Electric Vehicle Symp., Electric Vehicle Council 1, 375 (1969). Such lithium-molten salt batteries using sulfur positive electrodes when fully developed could provide an energy density of greater than 100 watt-hr/lb. Were a cycle life of 2500 cycles and an operating life of 10 years attainable with these batteries, they could satisfy all the requirements of electric power peaking, which is of great interest to the electric utility industry for providing off-peak energy storage and load leveling.
It has been found, however, that long cycle life is difficult to attain with such high-temperature molten salt batteries containing a sulfur electrode because of the gradual loss of the active sulfur material from the positive electrode compartment at these elevated temperatures. Sulfur loss generally occurs by vaporization of the sulfur or by dissolution of intermediate discharge products (polysulfide ions) in the molten salt electrolyte followed by diffusion from the positive electrode compartment through the bulk of the electrolyte to the negative lithium electrode.
To eliminate some of these problems, it has been proposed (U.S. Pat. No. 3,898,096, assigned to the assignee of the present invention) to use certain selected transition metal chalcogenides as positive electrode material in lieu of elemental sulfur. The preferred positive electrode materials are copper sulfide, iron sulfide, nickel sulfide, and nickel oxide. The patent teaches that the positive electrode materials, which are in solid form at the operating temperature of the molten salt electrolyte battery, must be made readily available in a finely divided form presenting a high specific surface.
Two approaches generally have been followed in the construction of a negative lithium electrode for use in an electrical energy storage device, such as a rechargeable battery, particularly one employing a molten salt electrolyte. In one approach, the lithium is alloyed with another metal such as, for example, aluminum to form a solid electrode at the operating temperature of the cell. In the other approach, liquid lithium is retained in a foraminous metal substrate by capillary action. Heretofore, the latter approach has been preferred because it offers higher operating cell voltages and therefore potentially higher battery energy densities. Certain problems are encountered, however, when it is attempted to retain molten lithium in a foraminous metal substrate. More particularly, most metals which are readily wetted by lithium are too soluble in the lithium to permit their use as the metal substrate, whereas most metals structurally resistant to attack by molten lithium are poorly wetted by the lithium when placed in a molten salt electrolyte.
Various other materials have been suggested for use as an alloy with lithium to form a solid electrode. In U.S. Pat. No. 3,506,490, for example, it is suggested that the lithium be alloyed with either aluminum, indium, tin, lead, silver or copper. However, none of these materials have been proven to be completely satisfactory. More particularly, these other suggested materials, such as tin and lead for example, form alloys containing lesser amounts of lithium than does aluminum, and thus have a still lower capacity (ampere-hours) per unit weight of alloy. Further, the potential of these other alloys compared with liquid lithium is more positive than that of a lithium-aluminum alloy; thus, alloys of such other materials are less desirable. Other patents relating to solid lithium anodes include U.S. Pat. Nos. 3,506,492 and 3,508,967.
More recently, in U.S. Pat. No. 3,969,139, assigned to the assignee of the present invention, there has been suggested an improved lithium electrode and an electrical energy storage device such as a secondary battery or rechargeable electrochemical cell utilizing such electrode. The improved electrode comprises an alloy containing lithium and silicon in intimate contact with a supporting current-collecting matrix. The lithium is present in the alloy in an amount from about 28 to 80 wt.%.
Various materials have been suggested for use as current collecting structures for electrodes in lithium-metal sulfide batteries. The particular material selected must of course be resistant to the corrosive environment at the various operating temperatures and electrical potentials. This is particularly true of the positive electrode structure. Molybdenum is particularly well suited for the component as it is highly conductive to electricity and reliably resists corrosion. However, it is prohibitively expensive and difficult to fabricate. Nickel, steel, copper and alloys thereof also have been suggested for use in such devices. However, it now has been found that these materials, while having a desired high electrical conductivity, are incapable of withstanding the corrosive environment encountered by the positive electrode for extended periods of time.
Copper and steel are quite rapidly attacked while it has been found that with continued cycling of the device, nickel appears to undergo some form of intergranular corrosion which renders it unsuitable for use where long life is necessary. Obviously, there is need for a suitable material for use as a positive electrode structure for use in such devices. Ideally, such material would be inexpensive, corrosion-resistant and have a low electrical resistivity.